Apple Computer was granted a patent for an enigmatically titled "Electronic device" Tuesday, May 10th, 2005. Illustrations for the device clearly indicate it to be a tablet-style Macintosh, and patent filings specifically compare it to the "HP Compaq Tablet PC" and several other tablet machines.

The patent was filed on March 17th, 2004 (first broken by The Register in August, 2004 as noted in the comments), and granted by the United States Patent and Trademark Office (USPTO) as US Design Patent No. D504,899 on May 10, 2005.

New in the issued patent is an illustration (see Figure 9 below) that demonstrates and confirms that the device is a handheld device with a touch screen.

The patent application noted that, "FIG. 9 is an exemplary diagram of the use of the electronic device thereof; [the] broken lines being shown [are] for illustrative [purposes] only, and form no part of the claimed design"

It's important to note that being granted a design patent does not necessarily mean that Apple has any intention of releasing a particular product, and Apple has filed for many design patents in recent years for products that did not make it to market.